Dirt: The Erosion of Civilizations - Kootenay Local Agricultural Society

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because it allows for free air circulation, good drainage, and easy access to
plant nutrients.
Clay minerals are peculiar in that they have a phenomenal amount of
surface area. There can be as much as two hundred acres of mineral surfaces
in half a pound of clay. Like the thin pieces of paper that compose a
deck of cards, clay is made up of layered minerals with cations—like potassium,
calcium, and magnesium—sandwiched in between silicate sheets.
Water that works its way into the clay structure can dissolve cations, contributing
to a soil solution rich in plant-essential nutrients.
Fresh clays therefore make for fertile soil, with lots of cations loosely
held on mineral surfaces. But as weathering continues, more of the nutrients
get leached from a soil as fewer elements remain sandwiched between
the silicates. Eventually, few nutrients are left for plants to use. Although
clays can also bind soil organic matter, replenishing the stock of essential
nutrients like phosphorus and sulfur depends on weathering to liberate
new nutrients from fresh rock.
In contrast, most nitrogen enters soils from biological fixation of atmospheric
nitrogen. While there is no such thing as a nitrogen-fixing plant,
bacteria symbiotic with plant hosts, like clover (to name but one), reduce
inert atmospheric nitrogen to biologically active ammonia in root nodules
2–3 mm long. Once incorporated into soil organic matter, nitrogen can
circulate from decaying things back into plants as soil microflora secrete
enzymes that break down large organic polymers into soluble forms, such
as amino acids, that plants can take up and reuse.
How fast soil is produced depends on environmental conditions. In 1941
UC Berkeley professor Hans Jenny proposed that the character of a soil
reflected topography, climate, and biology superimposed on the local geology
that provides raw materials from which soil comes. Jenny identified
five key factors governing soil formation: parent material (rocks), climate,
organisms, topography, and time.
The geology of a region controls the kind of soil produced when rocks
break down, as they eventually must when exposed at the earth’s surface.
Granite decomposes into sandy soils. Basalt makes clay-rich soils. Limestone
just dissolves away, leaving behind rocky landscapes with thin soils
and lots of caves. Some rocks weather rapidly to form thick soils; others
resist erosion and only slowly build up thin soils. Because the nutrients
available to plants depend on the chemical composition of the soil’s parent
material, understanding soil formation begins with the rocks from which
the soil originates.
skin of the earth